In many electronic systems, and particularly in data processing systems, there is always the need to provide regulated d-c voltages to the logic and other circuitry of the system. Typically, a switching regulated power supply is employed which receives raw d-c (which may have been rectified from a-c mains and filtered), chops this raw d-c (as by alternately enabling SCRs) at a rate much higher than the alternating rate of the mains (and typically ultrasonic), passes the chopped d-c through a step-down transformer (which, because of the switching frequency of the chopped d-c, need not be massive), rectifies the output from the step-down transformer and regulates and filters the rectified output to obtain a regulated output.
The regulation of switching power supplies is often achieved by feeding back the sensed output voltage to a control circuit in the switching apparatus itself. In switching regulated power supplies of this type, subtle (and even conspicuous) instabilities may occur in the form of oscillations resulting from the effects of feedback obtained from an output filter employing reactive (and thus phase shifting) components in conjunction with the operation of the regulator circuit. This condition (which can cause serious problems in the powered circuitry ranging from insidious intermittent errors to destruction) has been recognized and addressed in the prior art. See, for example, U.S. Pat. No. 4,298,924 (issued Nov. 3, 1981) to Luther L. Genuit, entitled SWITCHING REGULATOR WITH PHASE SHIFT SUBTRACTION, which describes a switching regulator incorporating the concept of employing a feedback voltage obtained by subtracting the phase shift voltage appearing across an output filter choke from the output voltage.
A more general approach has been to use attenuation-phase methods of rolling off the open loop gain of the regulator power amplifier to zero decibels with a phase margin of about 45.degree.. (This technique has sometimes incorporated a first derivative component into the feedback voltage.)
In my co-pending application Ser. No. 07/390,016, filed Aug. 7, 1989, and entitled CANCELLATION OF REGULATOR OUTPUT FILTER POLES BY SECOND DERIVATIVE FEEDBACK, very effective switching power supply stabilization method and apparatus are disclosed and it is shown that the introduction of second derivative feedback, in proportions to first and zero derivative feedbacks in accordance with the method, substantially eliminates the output filter (the essential insidious instability culprit in well-designed switching power supplies) from the power supply circuit characteristic equation a if it has been physically removed. For many switching power supplies, incorporation of my invention described and claimed in the aboveidentified co-pending application will virtually eliminate instability.
However, it has been found that, in certain switching power supplies under certain conditions, subtle transient instability may still occur even if my above-referenced invention has been incorporated into the regulator circuitry. It is thus to the end of determining the source of such residual instability and to its elimination that the present invention is directed.